Hydrothermal Synthesis of Hierarchical Nanocolumns of Cobalt

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J. Phys. Chem. C 2009, 113, 6566–6572

Hydrothermal Synthesis of Hierarchical Nanocolumns of Cobalt Hydroxide and Cobalt Oxide Yongzheng Shao, Jing Sun,* and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ReceiVed: January 19, 2009; ReVised Manuscript ReceiVed: February 23, 2009

In this paper, we report a facile method to prepare high-yield one dimensional hierarchical β-Co(OH)2 nanocolumns through one-step hydrothermal synthesis with hydrazine hydrate and Na3PO4 as morphologydirecting agents, which play important roles in the formation of hierarchical nanocolumn morphology. The hierarchical Co3O4 nanocolumns are obtained by the β-Co(OH)2 nanocolumns annealed at 400 °C for 4 h. The formation mechanism of hierarchical nanocolumns has been clearly demonstrated by studying morphology evolution processes of β-Co(OH)2 nanocrystals upon reaction time, which undergo two steps, the formation of nanorods and then growing sideways in the form of thin nanoplatelets to form hierarchical nanocolumns. Moreover the UV-vis spectra and magnetic properties of Co3O4 nanocolumns also have been investigated. Introduction In recent years, controllable synthesis of hierarchically structured materials via “bottom-up” techniques assembled by nanoscale building blocks has attracted intense attention, which is not only a useful tool for the realization of controllable synthesis technology, but also expected to explore their potential application in optics, mechanics, magnetics, electronics, catalysis, and biology because of their optimized properties such as large surface-to-volume ratio, full of nanotips or nanoplates, and multifunctions.1,2 Moreover, many studies have been developed to produce hierarchically structured materials, such as ZnO nanocombs,3 ZnO nanotowers,4 ZnO nanosaws,5 nanocolumns (ZnO6 and NiO7), and so forth. Cobalt hydroxides have attracted intense interest due to their many important applications. For example, cobalt hydroxide can be added as a component or additive to improve the electrochemical properties of nickel hydroxide electrodes,8 alkaline secondary batteries9 and β-Co(OH)2/zeolite molecular sieves.10 Moreover, β-Co(OH)2 has been investigated to use as reactive templates for highly textured thermoelectric cobaltite ceramics11 and as the precursor for the preparation of cobalt oxides nanomaterials by the thermal conversion.12 Co3O4 is a valuable material with applications in lithium batteries, gas sensing, catalyst, and electronics.13 Previously, numerous works have been developed to prepare various morphologies. Ohta et al. synthesized β-Co(OH)2 nanoplatelets by using poly(vinylpyrrolidone) (PVP) as the morphology-directing agent.14 β-Co(OH)2 nanowires12 and nanotubes15 were obtained by using ammonia aqueous solution as the alkaline and complexing reagent. Butterfly-like β-Co(OH)2 were prepared by ethylenediaminemediated synthesis.16 Also Co3O4 nanocubes were synthesized through a nitrate-salt-mediated formation route or surfactanttemplated fabrication.17 Co3O4 nanorods18 or nanotubes19 were fabricated through thermal conversion of rodlike precursor, as well as β-Co(OH)2 nanorod.20 Athough there are some reports about hierarchical structure of both β-Co(OH)2 and Co3O4, such * To whom correspondence should be addressed. Phone: +86-2152412718. Fax: +86 21 52413122. E-mail: (J.S.) [email protected]; (L.G.) [email protected].

as flowerlike structure21 and hierarchical spheres.22 However, there is no report about one-dimensional (1D) hierarchical structure of both β-Co(OH)2 and Co3O4 by using hydrazine hydrate as the morphology-directing agent in the synthesis of β-Co(OH)2 and Co3O4. Herein, we first report a facile method to prepare high-yield 1D hierarchical β-Co(OH)2 nanocolumns and hierarchical Co3O4 nanocolumns through one-step hydrothermal synthesis with hydrazine hydrate and Na3PO4 as morphology-directing agents. We have investigated the morphology evolution and formation mechanism clearly on the basis of TEM images of products obtained at different reaction time. Furthermore, the present study is helpful not only for realizing morphology-controlled synthesis, but also for controlling synthesis of new nanostructure. Experiment Section All chemicals were analytical grade and used without purification. In a typical synthesis of cobalt hydroxide hierarchical nanocolumns, 1 mmol of Co(NO3)2 · 6H2O and 9.5 mg Na3PO4 · 12H2O were dissolved in 70 mL of distilled water. After the solution was stirred for 0.5 h, 5 mL of hydrazine hydrate (85%) was slowly added, which caused a color evolution from red-pink to green. Then the whole mixture was transferred into a Teflon-lined autoclave maintained at 180 °C for 18 h. After the autoclave was cooled to room temperature, red-pink precipitates were collected and washed with distilled water and ethanol for four times, respectively. Then it was dried in a vacuum oven at 60 °C for 5 h. The obtained products were then annealed at 400 °C for 4 h in a furnace in an air condition to obtain cobalt oxides. Powder X-ray diffraction (XRD) was carried out on a Japan Rigaku D/max 2550V X-ray diffractometer using Cu KR (λ ) 0.15406 nm) radiation at 40 kV and 60 mA. Transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy (HR-TEM) images, and selected area electron diffraction (SAED) patterns of products were collected using a JEOL-2100F electron microscope with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were collected on a JSM 6700F field-emission scanning electron

10.1021/jp9005718 CCC: $40.75  2009 American Chemical Society Published on Web 03/30/2009

Hierarchical Nanocolumns of Cobalt Hydroxide and Cobalt Oxide

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6567 by the Brunauer-Emmet-Teller (BET) method based on the N2 adsorption (Micromeritics ASAP 2020).

Figure 1. The XRD pattern of the hierarchical β-Co(OH)2 nanocolumns prepared at 180 °C for 18 h (a), and annealed at 400 °C for 4 h (b).

microscope. FTIR spectrum of the samples was characterized by a Bruker Tensor 27 IR spectrometer by using KBr as dispersant. UV-vis absorption spectra analysis was performed on a Shimadzu UV-3101PC spectrophotometer with dry-pressed disk samples at room temperature. Magnetization was measured by a commercial Physical Properties Measurement System (PPMS, Quantum Design Inc.) under 2 T magnetic field at room temperature. The specific surface area of products was estimated

Results and Discussion Characterization of Hierarchical Cobalt Hydroxide Nanocolumns. Figure 1a shows the XRD pattern of the as-prepared hierarchical nanocolumns. All the diffraction peaks are well indexed to a pure hexagonal phase of β-Co(OH)2 according to JCPDS 30-443. The shape of the product obtained at 9.5 mg of Na3PO4 · 12H2O for 18 h in a hydrothermal system was further examined by SEM and TEM. Figure 2a shows the SEM images of the obtained products, from which it is obviously observed that most of the products are hierarchical nanocolumns with diameters ranging from 100 to 300 nm and lengths ranging from 1 to 2 µm. The inset of Figure 2a is a single hierarchical nanocolumn that is constructed by stacked nanoplatelets. Most nanoplatelets with ∼10-40 nm in thickness grow perpendicularly to the central stem and are almost parallel to each other. Also a few nanorods without any nanoplatelet on their surface were seen in Figure 2a (such as region A). Close observations on the end of the nanocolumns, as shown in Figure 2b, show that there exists some small platelets on the tip of the nanocolumns, indicating that the hierarchical nanocolumns are formed by growth of nanoplatelets from central stem or growth of additional nanoplatelets on the tips of preformed nanocolumns,6,23 but not assembly of the individual nanoplatelets.7,24

Figure 2. Morphology of hierarchical β-Co(OH)2 nanocolumns: (a,b) SEM images of products. (c) Typical TEM image of a single hierarchical β-Co(OH)2 nanocolumn and its SAED pattern. (d) The high-magnification TEM image of a single hierarchical β-Co(OH)2 nanocolumn from side view.

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Figure 3. Morphology of Co3O4 obtained by the β-Co(OH)2 nanocolumns annealed at 400 °C for 4 h. (a) TEM image of the hierarchical Co3O4 nanocolumns. (b) Typical TEM image of a single hierarchical Co3O4 nanocolumns and its SAED pattern. (c,d) SEM images of products.

The morphologies of the hierarchical β-Co(OH)2 nanocolumns were further characterized by TEM. From the typical TEM image of a single β-Co(OH)2 nanocolumn together with its SAED pattern (Figure 2c), the nanocolumn is a single crystal growing along the [001] direction. The streaks between bright spots indicate the existence of some defects, such as stacking faults. Combined with the SEM image of the single hierarchical nanocolumn, we find that the small nanoplatelets are oriented along the [001] direction. Figure 2d shows the high magnification TEM image of a single hierarchical β-Co(OH)2 nanocolumn from side view. The lattice fringe of the nanoplatelet and central stem is clearly observed, which further confirm that the nanocolumn is a single crystal structure. The interplanar spacings of the nanoplatelet (A) and the central stem (B) are both 0.461 nm, corresponding well to the d001 of β-Co(OH)2 crystal. It confirms that nanoplatelets grow vertically from the stem and have a size distribution that might be due to temperature distribution and concentration distribution of the solution. Characterization of Hierarchical Cobalt oxide nanocolumns. The XRD pattern of products obtained by the β-Co(OH)2 nanocolumns annealed at 400 °C for 4 h is shown in Figure 1b. All the diffraction peaks are well indexed to a pure cubic phase of Co3O4 according to JCPDS 9-418, indicating the formation of Co3O4. Figure 3a,b shows the TEM images of the hierarchical Co3O4, which have the same morphology and size as the former hierarchical β-Co(OH)2 nanocolumns without any change in shape. The oxidative reaction is described as follows: 6Co(OH)2 +O2f 2Co3O4 + 6H2O. They are constructed by nanoparticles

and many pores (Supporting Information, Figure S1a) because of the removal of water molecules at high temperature. The BET specific surface area of Co3O4 nanocolumns is 54.4 m2/g, and the average pore diameter calculated by using the desorption branch of the isotherm is 9.03 nm. From the SAED pattern of a typical single hierarchical Co3O4 nanocolumn shown in the inset of Figure 3b, they are polycrystalline. The several bright spots along [111] direction show that the overall nanocolumn stems are along the [111] direction and the small nanoplatelets are oriented along the [111] direction. Figure 3c,d shows the SEM images of the products; the morphology of product is well preserved, which indicates the hierarchical Co3O4 nanocolumns could be obtained with high yield through the thermal conversion of β-Co(OH)2. From Figure 3, no apparent change in diameter and size happened in the transformation from β-Co(OH)2 nanocolumns to Co3O4 nanocolumns. As investigated by Archer et al.,15 β-Co(OH)2 could be transformed into needlelike Co3O4 nanotubes without apparent increase in diameter, because there is no larger displacement caused by the rearrangement of Co atoms in the brucite-like sheets. Formation of Cobalt Hydroxide. In order to investigate the formation mechanism of the hierarchical β-Co(OH)2 nanocolumns, detailed time-dependent experiments were carried out at 180 °C. From the TEM images of these products shown in Figure 4a-e, the morphology evolution of the hierarchical β-Co(OH)2 nanocolumns is observed clearly. At the beginning, when the reaction time is 30 min (Figure 4a), some blocks with some short nanorods on their surfaces are obtained with the

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Figure 4. TEM images of the products obtained at different reaction times: (a) 0.5, (b) 1, (c) 6, (d) 12, and (e) 18 h. The inset is the corresponding magnified image.

diameter about 1 µm and length about 0.5 µm. When the reaction time is increased to 1 h, the blocks gradually diminish while the nanorods gradually increase in length and diameter. The main products are the blocks with long nanorods on their

surfaces, and the mean length of these blocks is decreased to 100 or 200 nm. And there are also a few nanorods dispersed randomly in the solution. From the magnified TEM image of the typical morphology of blocks (the inset of Figure 4b), it

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Figure 5. Schematic illustration of the formation mechanism of hierarchical Co(OH)2 nanocolumns.

seems that some nanorods grow out of center of the blocks at the both sides. Then prolonging the reaction time to 6 h, all the blocks disappear and only nanorods exist in the solution, which are about 200 nm in diameter and 1 µm in length (Figure 4c). From Figure 4c-1 and c-2, we find that the surfaces of some nanorods are rough, and there are many platelet-like protuberances which grew perpendicularly to [001] direction on the surface of nanorods. When the reaction time is 12 h, the hierarchical nanocolumns derived from the former rough nanorods take shape and become the main morphology of products, as shown in Figure 4d. The other products are nanorods with smooth surfaces, as well as the final products (region A of Figure 2a), from which we exclude the formation mechanism of assembly of the individual nanoplatelets. After prolonging the reaction time to 18 h, as shown in Figure 4e, the hierarchical nanocolumns become more uniform and perfect, where is no obvious variation in diameter and length of the central stem of hierarchical nanocolumns, which further demonstrates that our products are not formed by the growth of additional nanoplatelets on the tips of preformed nanocolumns. In our experiment, the red-pink Co(NO3)2 solution turned to green when the hydrazine hydrate was added, because the [Co(N2H4)3]2+ complexes were formed, which were very stable in alkaline system at room temperature. But with the increase of the reaction temperature, the complex began to react with OH- and formed Co(OH)2 nanocrystals gradually. The chemical reaction for the formation of Co(OH)2 can be expressed as follows

Co2+ + 3N2H4 ) [Co(N2H4)3]2+

(1)

N2H4 + H2O ) N2H5+ + OH-

(2)

[Co(N2H4)3]2+ + 2OH- ) Co(OH)2 + 3N2H4

(3)

On the basis of the above time-dependent morphology evolution stages, we conclude that the morphology evolution mainly undergo two steps, the formation of nanorods and the hierarchical nanocolumns, as illustrated in Figure 5. The first growth step can be explained as a nucleation-dissolutionrecrystallization mechanism.25 At the beginning, some small metastable blocklike precursor was first formed immediately with the increase of the reaction temperature, which was attributed to their intrinsic lamellar structure and coordinate to hydrazine hydrate.26,27 At the same time, many rodlike Co(OH)2 protuberances (pink area in Figure 5) were recrystallized on the

Figure 6. FTIR spectra of Co(OH)2 nanorods prepare at 180 °C for 6 h.

Figure 7. UV-vis absorption curves of Co3O4 nanocolumns (a) and nanorods (b) obtained by the Co(OH)2 annealed at 400 °C for 4 h.

surfaces of the former blocklike precursor (dark red area in Figure 5). Subsequently, they grew larger to form nanorods at the effect of Na3PO4 [Step A]. On the contrary, the former metastable blocklike precursor gradually dissolved in the solution because of alkaline condition and provided Co ions to form nanorods (reaction 3). The presence of Na3PO4 is the crucial reason to recrystallize as nanorods, confirmed by further experiment in the absent of Na3PO4 in which we could only obtain Co(OH)2 nanoplatelets because the blocks condensed and formed hexagonal nanoplatelets through Ostwald ripening (Supporting Information, Figure S2). As we know, the mineralizers have obvious influence on the morphology and size of the product. It has been reported that phosphate ions have a morphology-directing effect in the synthesis of hematite nanotubes and other nanostructure because of the selective adsorption of phosphate ions on hematite nanocrystals.28 They reported that phosphate ions prefer to adsorb to the planes perpendicular to (001) planes as an inhibitor to decrease the rates of crystal growth, resulting in growth along [001] direction. Here, phosphate ions also have a morphology-directing effect in the formation of Co(OH)2 nanorods, which is similar to their reports. We investigated by the FTIR spectrum of the obtained nanorods, as shown in Figure 6, the peak at 1030 cm-1 was assigned to P-O stretching and bending vibration,29 indicating the adsorption of phosphate ions on the surfaces of Co(OH)2 nanorods because they could not be washed completely by water or ethanol. Further research on the detailed inter-reaction between certain Co(OH)2 plane and phosphate ions is still going on. With the increase in reaction time, these nanorods became longer; on the other hand, the blocklike precursor were diminishing

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Figure 8. Hysteresis loops of the Co3O4 nanorods (a) and hierarchical nanocolumns (b) at room temperature.

[Step A]. Finally, the blocks with many nanorods were split into individual nanorods after the blocklike precursor was dissolved completely [Step B]. After the formation of nanorods, with the increasing of the hydrothermal treatment time, many small platelet-like protuberances appeared on the surfaces of nanorods at the effect of hydrazine hydrate and a gas-liquid equilibrium in the autoclave, which provided many high-energy sites for nanocrystals growth [Step C].25 Then Co ions spontaneously nucleated onto these active sites to form nanoplatelets. Hydrazine hydrate has been proven to have structure-directed effect in the formation of lamellar structure materials of Co(OH)2 (Supporting Information, Figure S2), Ni(OH)2,7 and Mg(OH)2,27 because of its intrinsic lamellar structure and the bidentate ligand of hydrazine hydrate. So, in our case it prefers to form nanoplatelets when Co(OH)2 nucleated onto the small protuberance of the side of former nanorods at the effect of hydrazine hydrate. Furthermore, only nanoplatelets could be obtained but not nanocolumns when we used ammonia instead of hydrazine hydrate under the same conditions (Supporting Information, Figure S3). Finally, the hierarchical nanocolumns were formed as the nanoplatelets grew longer [Step D], and became more uniform and perfect through Ostwald ripening by prolonging the reaction time. To investigate the influence of the different parameters on the morphology of the Co(OH)2 product, some designed experiments were carried out. For example, the concentration of phosphate ions influenced the final morphology of product remarkably; when the amount of Na3PO4 · 12H2O was less than 4 mg, only spherelike particles could be obtained (Supporting Information, Figure S4). On the other hand, a small amount of nanocolumns and many other shaped nanoparticles could be obtained when the amount of Na3PO4 · 12H2O was above 19 mg (Supporting Information, Figure S5). The influence of pH value was also investigated (about 9 in our synthesis system); when the pH value was turned to 12, only nanoplatelets were obtained (Supporting Information, Figure S6). On the contrary, the reaction would not occur without hydrazine hydrate when the pH value was about 7. Furthermore, we used NaOH instead of hydrazine hydrate when the pH value was about 9; as a result, there were no hierarchical Co(OH)2 nanocolumns observed, which confirmed the morphology-direction effect of hydrazine hydrate. The UV-vis Spectra of Co3O4 Nanocrystals. The UV-vis spectra of Co3O4 nanorods and hierarchical nanocolumns obtained by the corresponding Co(OH)2 nanorods (obtained at 6 h) and hierarchical nanocolumns (obtained at 18 h) annealed at 400 °C for 4 h were illustrated in Figure 7. The curves exhibit different absorbance edges, 731 nm for the Co3O4 hierarchical nanocolumns (a) and 743 nm for the Co3O4 nanorods (b), which is referred to the ligand-metal charge transfers O II fCoIII, compared with the absorbance edge at more than 800 nm of 47 nm nanocubes report by Zeng et al.30 The corresponding

absorbance edge is blue-shifted remarkably, which is mainly caused by the quantum size effect of nanoparticles of these architectures. As shown in the high magnification TEM images of Co3O4 nanorods (Supporting Information, Figure S1b) and hierarchical nanocolumns (Supporting Information, Figure S1c), they are both made up of tiny nanoparticles about 5 nm in size and very porous, which result in the tiny nanoparticles of these nanorods or nanocolumns that are in or near the quantum confined regime.31 Furthermore, in our experiment the absorbance edge for the Co3O4 hierarchical nanocolumns shows that it is obviously blue shifted compared with the Co3O4 nanorods. As shown in Supporting Information, Figure S1a-b, nanoparticles of nanorods are the same size as with those of the central stems of nanocolumns, but the particles size of side-nanoplatelets is also about 5 nm, which caused the additional quantum size effect of the Co3O4 hierarchical nanocolumns to make the absorbance edge for the Co3O4 hierarchical nanocolumns blue shifted. Magnetic Study. Figure 8 shows the magnetic properties of the Co3O4 nanorods (Figure 8a) and hierarchical nanocolumns (Figure 8b). The shapes of these magnetic curves are very similar to those of porous nanotubes of Co3O4 measured at 4.2 K32 and nanowires of Co3O4 measured at room temperature.33 From these figures, a low coercive force and remanent magnetization can be seen, which indicate the Co3O4 nanorods and hierarchical nanocolumns both exhibit a little ferromagnetic property. But the coercive force of nanorods (Hca ) 117 Oe) is lower than that of hierarchical nanocolumns (Hcb ) 123 Oe). The surface spins may be the main reason of the hysteresis,34 although more surface spins get free with temperature increased to room temperature, there exist some uncompensated spins on the surface. The larger surface area of the hierarchical nanocolumns (a BET surface area of 54.4 m2/g) than that of nanorods (a BET surface area of 50.4 m2/g) increases the uncompensated spins of surface, which leads to an increase of the coercive force and the horizontal shift of the hysteresis loop. The larger shift of the hysteresis loop of the hierarchical nanocolumns indicates the unidirectional anisotropy of nanorods (a) is higher than the hierarchical nanocolumns (b). Under the applied field of 2 T, the magnetization is about 0.7 emu/g of nanorods and 0.6 emu/g of hierarchical nanocolumns at room temperature; these are a little larger than 0.45 emu/g for 70 nm particles at same conditions, because some of blocking nanopaticles grew into larger crystals in the annealing process and also possibly due to the macroscopic magnetic quantum effects of the other tiny blocking nanoparticles with ∼5 nm in size (Supporting Information, Figure S1a-c).36 Conclusion In summary, we have developed a one-step hydrothermal synthesis method to prepare high-yield hierarchical β-Co(OH)2

6572 J. Phys. Chem. C, Vol. 113, No. 16, 2009 nanocolumns without any surfactant or template. The morphology evolution of the hierarchical nanocolumns has been investigated by changing the reaction time. We find that they undergo two steps, the formation of nanorods and then grow sideways in the form of thin nanoplatelets to form hierarchical nanocolumns. Hydrazine hydrate not only acted as the reactant but also acted as the morphology-directing agent to play an important role in the formation of hierarchical nanocolumn morphology, and Na3PO4 helped to form nanorods in the first growth step. It is very useful for the controllable synthesis of new nanostructures. The hierarchical Co3O4 nanocolumns are obtained by the thermal conversion of β-Co(OH)2. They exhibit the blue-shifted absorbance edge because of the quantum size effect of their constructed tiny nanoparticles; the hierarchical Co3O4 nanocolumns also exhibit an enhanced coercive force more so than the nanorods because of the larger surface-tovolume ratio. Moreover, they are expected to have applications in the functional photoelectric nanodevices and biology. Acknowledgment. This work was financially supported by the National Key Project of Fundamental Research (No. 2005CB6236-05), the National Natural Science Foundation of China (No. 50672112), and the Hundred Talent Program. Special thanks are given to Dr. X. F. Song and S. W. Yang for their discussion on research works. Supporting Information Available: TEM images of β-Co(OH)2 obtained at designed conditions and the high magnification TEM images of Co3O4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Zheng, H. C. J. Mater. Chem. 2006, 16, 649. (b) Li, M.; Schnablegger, H.; Man, S. Nature (London) 1999, 402, 393. (c) Shi, H. T.; Qi, L. M.; Ma, J. M.; Wu, N. Z. AdV. Funct. Mater. 2005, 15, 442. (d) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Yuan, X. L.; Sekeguchi, T.; Golberg, D. AdV. Mater. 2005, 17, 971. (2) (a) Zhou, X. F.; Chen, S. Y.; Zhang, D. Y.; Guo, X. F.; Ding, W. P.; Chen, Y. Langmuir 2006, 22, 1383. (b) Ostermann, R.; Li, D.; Yin, Y. D.; McCann, J. T.; Xia, Y. N. Nano Lett. 2006, 6, 1297. (c) Cho, S. O.; Lee, E. J.; Lee, H. M.; Kim, J. G.; Kim, Y. J. AdV. Mater. 2006, 18, 60. (3) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (4) Xu, F.; Yu, K.; Li, Q.; Zhu, Z. Q. J. Phys. Chem. C 2007, 111, 4099. (5) Shen, G.; Chen, D.; Lee, C. J. J. Phys. Chem. B 2006, 110, 15689. (6) Zhang, T.; Dong, W.; Keeter-Brewer, M.; Konar, S.; Njabon, R. N.; Tian, Z. R. J. Am. Chem. Soc. 2006, 128, 10960. (7) (a) Coudun, C.; Hochepied, J. F. J. Phys. Chem. B 2005, 109, 6069. (b) Zhu, J. X.; Gui, Z.; Ding, Y. Y.; Wang, Z. Z.; Hu, Y.; Zou, M. Q. J. Phys. Chem. C 2007, 111, 5622. (8) Li, W. Y.; Zhang, S.; Chen, Y. J. J. Phys. Chem. B 2005, 109, 14025.

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